I have started to learn about blockchain technologies, BTC and other crypto-currencies and right now interested in mining.
I know that mining of BTC is not very profitable right now, but other currencies can be profitable.

I looked at ETH, DASH and Monero and according to this site I can earn near 150K per day with AntMiner S7, which seems to be unrealistic.

Please help me understand how this stuff works, is it real or have I some misunderstood something?

Well, BTC limitation is not a true. But yes, sha256 world is not quite big.
Mining btc on antpool will give you basic statistic, it's depends on your local costs and power limits more than the your Antminer can do)

Ok, I have custom build S5 with 4 hash boards that can provide ~2Th on freq@275(this is my own limitations).

Okay back to PnL, so I'm able to mine ~0.001/day BTC on antpool.
Then I moved to multipool and started mining alt coins that provided much more profits for me, depends on coin ~ ~0.002-0.004/day BTC
https://www.multipool.us/dashboard/pools/sha256

You may try to find another alt coin pools, but keep in mind your antminer works with SHA-256 algorithm only.